1. Field of the Invention
The present invention relates to a method for the preparation of metal matrix fiber composites. In particular, the present invention relates to a method which produces metal powders uniformly coated on fibers as a result of aerosolization of the powders and then consolidation of the powder on the fibers to form the matrix.
2. Description of Related Art
Fabricating metal matrix composites with fiber tows surrounded by the metal matrix has always presented difficulties to materials producers. Unlike the viscous polymers, liquid metals have a viscosity similar to water. (Mortensen, A., et al, Journal of Metals, 30 (1986)). If the fiber can be wetted by the matrix material, a liquid-infiltration technique could be a first choice because of simplicity and continuity. If the fiber is not wetted by the metal, a suitable fiber coating or matrix alloying addition had to be found to facilitate wetting. In either case, interfacial reaction between the metal and the fiber is hard to control due to overexposure to molten metal. Uneven fiber distribution in the metal matrix is also an unsolved problem. The problems encountered with liquid phase processes are 1) porosity from solidification shrinkage (opening voids between the fibers), 2) low fiber volume fraction, 3) interface reaction degradation, and 4) uneven distribution of fibers. Most of the problems arise from the difficulty in wetting the fiber with the liquid metal.
The problems are reduced with squeeze casting into a mold with a preform of fibers (Bader, M. G., et al., Composites Science and Technology 23 287-301 (1985); and Kohara, S., et al., Composites '86: Recent Advances in Japan and the United States, eds. K. Kawata, S. Umekawa and A. Kobayashi, (Proceedings of Japan-U.S. CCM-III, Tokyo, 491-496 (1986)). However the problems increase as the fiber diameter decreases. Alloy additions can reduce the wetting contact angle with the fibers; however, they also cause more interface reactions, which usually degrades the bond or the integrity of the fiber (Mortensen, A., et al., Journal of Metals, p. 30 (March 1986)). Other methods, such as electroplating, spraying, chemical vapor deposition and physical vapor deposition, could produce high quality composites, but the methods are time consuming and expensive. Plasma spraying coats fibers with a liquid metal, which can later be arranged in a desirable way, can be accomplished but only with large (140 .mu.m) diameter plasma sprayed fibers. Furthermore, these known techniques are generally not suitable for commercial large-scale or continuous processing.
Powdered metal processing with fibers eliminates or reduces the interface wetting/reaction problem with liquid processing. The metal is sintered and forms around the fiber in the solid state. The kinetics for interface reactions are much slower in powder methods. The two major problems of this strategy are 1) fiber damage may occur under the pressure needed for consolidation (Erich, D. L., Int. J. Powder Metallurgy, 23 45-54 (1987), and 2) high fiber volume fractions are not possible, if large or agglomerated powder particles are present, since they cause the fibers to bunch up (Shimizu, J., et al., Metal & Ceramic Matrix Composites: Processing Modeling & Mechanical Behavior, eds. R. B. Bhagat, A. H. Clauer, P. Kumar and A. M. Ritter, (TMS/AIME Warrendale Pa.) 31-38 (1990)).
Fibers can be manually arranged between layers of foil and hot pressed. There are a limited number of foil compositions available and the volume fraction of fibers is often small, and the fiber diameters are large (Mortensen, A., et al., Journal of Metals, p. 30 (March 1986)). These processes often provide dramatically better properties than predicted by continuum models of discontinuous fibers, since dislocations generated near the interface deflect cracks and change matrix properties near the interface, due to strains from thermal expansion mismatch (Erich, D. L., Int. J. Powder Metallurgy, 23 45-54 (1987); and Arsenault, R. J., Mat. Sci. and Eng. 64 171-181 (1984)).
A continuous fiber-reinforced polymer matrix composite method was originally developed by Drzal et al (U.S. Pat. Nos. 5,042,122, 5,042,111, 5,123,373, 5,128,199, and 5,310,582). In the Drzal et al method, an unsized carbon fiber tow goes through different chambers to make a prepreg tape of a polymer matrix composite. A fiber tow is driven by a motor from a fiber spool to pass above a speaker. The sound waves coming off the speaker spread the fibers apart. The spread fibers are held in position by ten stainless steel shafts spaced one inch apart and placed on the top of the speaker. After spreading, the fibers pass through an optional pre-treatment chamber to modify the fiber surface or to apply a thin coating of binder material to improve adhesion with the matrix. Then, the fibers enter an impregnation chamber, called aerosolizer, where small polymer particles (about 10 microns in diameter) are suspended by the effect of a vibrating rubber membrane placed on top of a speaker, which works as a bed of polymer powders. The powders are attached to the fibers by an electrostatic force generated from the static charges held by the fine polymer particles. After coating with polymer particles, the fibers pass through the oven chamber for about 15 seconds. The particles are heated by convection and radiation until sintering occurs between adjacent particles to form a thin film. The impregnated fibers are then cooled and wound on a take up drum. After a run, the resulting prepreg tape is cut into pieces to a desired length and are laid-up in a rectangular stainless steel mold for hot pressing according to a pressure-temperature-time profile. A sheet of continuous fiber-reinforced polymer matrix composite material is thus formed and is evaluated. The problem is to provide a continuous fiber metal matrix composite (CFMMC).
Finely divided metal powders are explosive in an atmosphere containing any oxygen and thus the aerosolization of powders in air has not been considered to be useful as a method for coating fibers. Serious problems are created by the use of aerosolized powders which have not been solved by the prior art.